It is already known that the ambient air pressure (ambient pressure) influences the behavior of the internal combustion engine. To allow for this influence, the information regarding the ambient pressure is, for example, measured by means of a suitable ambient-pressure sensor and transmitted to an engine control unit with a program-controlled processing unit. In the engine control unit, a suitable correction factor for the ambient pressure is calculated from the measured pressure values, so that engine actuators, for example injection valves etc., can be controlled as necessary relative to the ambient pressure. The disadvantage is that the ambient pressure has to be measured by means of a separate ambient-pressure sensor.
In order to avoid the use of an ambient-air pressure, which has to be additionally installed, the ambient pressure can be determined from existing measured and model values or from pressure values of a pressure sensor installed in the intake manifold and at a specific operating state of the engine. For example, the air pressure in the intake manifold is equal to ambient pressure with the engine stationary. The intake manifold pressure in this case is measured by means of a pressure sensor which is arranged at a suitable point in an intake manifold, downstream of a throttle valve.
It is furthermore known in an aspirated engine operating at full load the intake manifold pressure differs from the ambient air pressure. This pressure difference is, however, dependent on the air mass flow in the intake manifold and is essentially equal to the pressure drop at the air cleaner, which can be easily determined and stored as a curve. Therefore the current ambient pressure can be inferred from a known pressure drop at the air cleaner and air mass flow in the intake manifold or from the intake manifold pressure. However, this method can be used only for aspirated engines which do not have a compressor (turbocharger). With a turbocharged engine, with which the air mass flow in the induction tract is controlled by a charger, the aforementioned method can be used to determine the ambient pressure only with the engine stationary. This method cannot be used for turbocharged engines under high load because due to the charger the intake manifold pressure differs from the ambient pressure depending upon the operating state of the engine. Therefore, in this case there is no mathematical or physical relationship between the intake manifold pressure and the ambient pressure to be determined.
A further problem is that the ambient pressure can change very quickly when ascending or descending a hill. For this reason, it is desirable for the ambient pressure to be continuously matched and/or determined. To solve this problem, a further pressure sensor, which directly measures the ambient pressure, is used on known turbocharged engines. An ambient pressure sensor of this kind is, however, undesirable because it not only causes extra costs during the manufacture of the vehicle but as a fault source it can have a negative influence on the engine control.